The present invention relates to detents for optical fibre members. More particularly, the present invention relates to detent devices providing strain relief for an optical fibre member incorporating at least one optical fibre and at least one flexible tensile strength member.
Detent devices serve to provide axial strain relief for cables (the term xe2x80x9ccablesxe2x80x9d is meant to include both copper cables and optical fibre members). That is, when a cable is pulled, the detent device ensures that the pulling force does not act upon the conductor which may be pulled loose from a connector. In particular in the case of optical fibre cables, the relatively brittle optical fibres should not be pulled as they easily break. An example of an optical fibre detent device is described in U.S. Pat. No. 5,166,997 to Norland et al.
Optical fibre members are often provided with a strength member. The optical fibre member then comprises one or more optical fibres, one or more strength members, and a sheath. The strength member is preferably made of a flexible material having a high pulling strength. Often KEVLAR(copyright), a commercially available aramid fibre, is used for this purpose. The detent device causes any pulling force exerted on the optical fibre member to act upon the strength member and not on the optical fibre(s).
The detent device of U.S. Pat. No. 5,166,997 has an inner engagement member around which the threads of the strength member can be wrapped back. A band of elastomeric material is then placed over the wrapped portion of the strength member. The band rests in a groove in the outer surface of the inner engagement member. The strength member is thus affixed to the inner engagement member. Subsequently, a washer acting as a pressure plate is placed against the inner engagement member and is secured by a screw threaded casing, thus preventing any movement of the inner engagement member.
Although this known device provides a good strain relief, its application is relatively complicated and time-consuming. In addition, the device consists of a large number of parts, making it relatively expensive.
German Patent Application DE 3 640 695 discloses a detent device for optical fibre cables. The device comprises two substantially identical conical tubular parts which can move relative to each other to engage the strength member of an optical fibre cable and which are together held in a housing. The cable has to be threaded through the tubular parts to be accommodated in the device.
French Patent Application FR 2 747 201 discloses optical fibre cables and cable clamps for clamping such cables. A mechanism for clamping the strength member of an optical fibre cable is not disclosed.
It is an object of the present invention to avoid these and other disadvantages of the Prior Art and to provide a detent device which is easy to install and which does not require the optical fibre member to be threaded through the device.
It is another object of the present invention to provide a detent device which is economical.
It is a further object of the present invention to provide a detent device having a minimal number of components.
To meet these and other objects, a detent device as defined in the preamble is according to the present invention characterised in that the inner engagement member is movably received in the outer engagement member, in that the inner engagement member comprises at least one boss for wrapping back strength members, the at least one boss providing an outer wall of the said engagement member for clamping at least one strength member, and in that each boss is provided with a retaining tab for retaining one or more strength members.
The present invention is based upon the insight that the tapered arrangement of the engagement members is self-tightening. That is, the strength member wrapped back around the inner engagement member will close any gap between the engagement members when pulled, thus securing itself. There is no need for retaining bands or pressure plates, which reduces the number of components. Also, not having to apply a number of components facilitates the installation of the detent device.
It is noted that International Patent Application WO 98/35415 (Raychem) discloses a self-tightening cable clamp having a sliding part. However, this known device clamps the outside of the cable, thus exerting pressure on the cable body. Although slits are provided for securing flexible strength members of optical fibre cables, no means are provided for wrapping back and clamping these flexible strength members. Axial pull relief is in particular provided by securing a central strength member by an axial screw driven jaw member.
To facilitate the application of the strength member(s) the design of the detent device is such that the inner engagement member comprises at least one boss for wrapping back strength members, the at least one boss providing an outer wall of the said engagement member for clamping at least one strength member. Such a boss preferably extends perpendicularly to the direction of the optical fibre member. To avoid the strength members slipping off the boss and to make the installation of the device easier, each boss is provided with a retaining tab for retaining one or more strength members.
The detent device of the present invention can be arranged to accommodate a single optical fibre member. However, a very compact termination of a plurality of optical fibre members may be obtained when the inner engagement member is arranged for accommodating at least two optical fibre members.
As the inner engagement member may be loosely accommodated in the outer engagement member, preferably a lid is provided for closing off the outer engagement member so as to retain the inner engagement member and any optical fibre member accommodated therein. Advantageously, the lid is provided with at least one stabilising slot for receiving any stabilising fingers protruding from the outer engagement member so as to prevent any outward flexing of the outer engagement member. Such an arrangement allows much higher tensile forces as the shape and size of the outer engagement member are maintained.
Advantageously, the lid is provided with a plurality of stabilising slots for stabilising fingers. In addition, the outer engagement member may be provided with securing clamps for lockingly securing the lid.
A reduction of the number of components and an easier installation are obtained when the lid is integral with the outer engagement member.
Instead of, or in addition to the lid, a substantially U-shaped clip may be used to hold both the inner member and the walls of the outer member in their respective positions. Such a clip is preferably made of metal and may substantially enclose part of the outer engagement member.
The accommodation of two or more optical fibre members may be achieved in several ways. In a first embodiment, the inner member comprises two parallel bosses. That is, when two optical fibre members are used, each such member has its own boss. In a second embodiment, the inner engagement member comprises a single boss. That is, the single boss is shared between optical fibre members. In the latter embodiment, the boss is preferably elongate and has retaining tabs at both ends.
The inner engagement member could be loosely received in the outer engagement member so as to allow movement in virtually all directions. Preferably, however, guiding means are provided for guiding the movable inner engagement member in the outer engagement member. The guiding means may also retain the inner engagement member.
Preferably, the guiding means comprise a protrusion receivable in a slot in the inner engagement member. Advantageously, the protrusion is shaped so as to retain the inner engagement member, for example by being widened beyond the slot.
To maintain the locking position of the inner engagement member in the aperture the detent device of the present invention is preferably designed such that the inner engagement member is provided with at least one interlocking protrusion for interlocking with a serrated surface of the aperture. The protrusion and serrations are advantageously shaped so as to allow movement in the engaging direction and to resist movement in the disengaging direction of the device.
The invention also provides a kit-of-parts for forming a detent device as described above, as well as an optical fibre organiser provided with a detent device as described above.
In summary, the present invention provides a wrap-around detent device for optical fibre members incorporating at least one optical fibre and at least one flexible tensile strength member, the device comprising a casing and a detent means movably receivable within the casing, the casing and/or the detent means having a tapering surface and being arrangeable in use to form a gap between them such that:
(a) an optical fibre member can be inserted laterally into the casing so that its optical fibre(s) extend through the casing in a first direction,
(b) the strength member of the optical fibre member can be wrapped back around the movable detent means to extend into (preferably through) the said gap in a second direction at least different from (preferably opposite to) the said first direction, and
(c) tension applied to the optical fibre member in the said second direction moves the detent means so as to narrow the said gap and grip (or grip more tightly) the strength member therein, thus resisting transmission of the applied tension to the optical fibre(s) themselves.